1. Field of the Invention
The present invention relates to a method and apparatus for degassing a coating liquid, and in particular, to a method and apparatus for degassing a coating liquid, which is suitable for degassing a photosensitive material coating liquid, a heat development photosensitive material coating liquid, a transparent heat-sensitive material coating liquid, an organic solvent coating liquid, or the like.
2. Description of the Related Art
In general, in a coating liquid used to coat a support with various liquids, air bubbles are likely to be generated during an operation such as agitation, dispersion, or dispensation which is performed before a coating step. When the coating liquid containing air bubbles is supplied to a coating apparatus, then coated on a support using the apparatus, and dried, bubble defects such as bubble traces or pin holes are likely to occur in the coat film. Consequently, a good and uniform coat film cannot be formed. Therefore, a sufficient degassing process must be executed before the coating step. As a degassing method employed for such a purpose, the use of ultrasonic waves is well known. A large number of proposals for this technique have been made (Japanese Patent Application Publication Nos. 1984-92003, 1988-178807, 1993-92103, 1994-130547, 1996-318102, 1999-262601, 1999-197406, and 1999-290611).
Japanese Patent Application Publication No. 1984-92003 is a proposal that improves the arrangement of ultrasonic vibrators. Japanese Patent Application Publication No. 1988-178807 is a proposal that dissolved air be removed before irradiation with ultrasonic waves. Japanese Patent Application Publication No. 1993-92103 is a proposal that improves the configuration of an ultrasonic degassing apparatus. Japanese Patent Application Publication No. 1994-130547 is a proposal that a vacuum pump be used to reduce pressure before irradiation with ultrasonic waves and that a pressurization pump be used to increase the pressure after the irradiation with ultrasonic waves. Japanese Patent Application Publication No. 1996-318102 is a proposal that is a combination of reduced pressure ultrasonic degassing, pressurized ultrasonic degassing, and management of a liquid temperature. Japanese Patent Application Publication No. 1999-262601 is a proposal that is a combination of a plurality of degassing steps each combined with irradiation with ultrasonic waves. Japanese Patent Application Publication No. 1999-197406 that two degassing baths are provided in series having different conditions for irradiation with ultrasonic waves. Japanese Patent Application Publication No. 1999-290611 is a proposal that the oscillation frequency of ultrasonic waves is continuously modulated to attenuate or eliminate a standing wave to facilitate degassing.
However, there remain problems that cannot be solved by the various proposals using ultrasonic waves. The problems are as follows.
1) The air bubbles in the coating liquid cannot be sufficiently removed.
2) The gas dissolved in the coating liquid is precipitated to form air bubbles again.
3) The coating liquid is damaged or piping through which the coating liquid is transferred is corroded.
4) Ultrasonic waves applied must be optimized in accordance with the composition of the coating liquid or a degassing speed.
5) A combination of supersonic waves with another degassing method results in an increase in the size of the apparatus.
These problems will be described below. First, the problem 1) will be described.
A coating liquid such as a photographic sensitive coating liquid which contains various additives is originally likely to become frothy and is prone to contain air bubbles. Further, in recent years, with improvements in coating methods and productivity, there has been an increase in the viscosity of the coating liquid and in degassing throughput per unit time. Accordingly, although various degassing methods and apparatuses have been proposed, these conventional methods still have an insufficient degassing capability.
One of the reasons for the insufficient degassing capability is that when a liquid accommodated in a degassing container is irradiated with ultrasonic waves at a fixed oscillation frequency, ultrasonic vibration causes a fixed compressional wave to be formed. Thus, a standing wave is generated. The standing wave creates problems; air bubbles in the liquid are trapped in a particular area to prevent the bubbles from floating, or the standing wave degrades the capability of dissolving and eliminating fine air bubbles. As a result, air bubbles cannot be removed effectively.
To solve this problem, for example, Japanese Patent Application Publication Nos. 1984-92003, 1993-92103, and 1999-197406 propose configurations in which a plurality of ultrasonic vibrators are arranged on the bottom and sides of a degassing container and opposite one another. In this configuration, ultrasonic waves applied by the individual ultrasonic vibrators are complementary to one another. However, these configurations can increase the quantity of ultrasonic waves applied per unit area of the degassing container to improve degassing efficiency, but fail to eliminate the standing wave. It is thus difficult for these proposals to efficiently and perfectly remove air bubbles from the liquid.
To solve this problem, Japanese Patent Application Publication No. 1999-290611 and the like propose that the oscillation frequency of ultrasonic waves applied is continuously modulated over a predetermined frequency bandwidth around a frequency used as a reference to periodically continuously vary the distribution of sound pressure of ultrasonic waves applied to a coating liquid. This configuration has substantially solved the problem that bubbles in the coating liquid cannot be sufficiently removed. However, the degassing achieved by this configuration may be insufficient depending on the physical properties of the liquid and a degassing process speed per unit time. Specifically, The output of ultrasonic waves for each physical property of the liquid must be individually set in view of the degassing process speed per unit time.
Now, the problem 2) will be described. In the conventional ultrasonic degassing apparatus, a coating liquid is accommodated in the degassing bath and then irradiated with ultrasonic waves. When this apparatus is used to irradiate the coating liquid with ultrasonic waves, the pressure of the coating liquid varies to facilitate the dissolution of air bubbles in the liquid under pressure. This effect is further enhanced by increasing the output of ultrasonic waves. However, an excessive increase in the output of ultrasonic waves causes fine air bubbles of negative pressure, that is, cavitation to be instantaneously generated in the liquid and then disappear. When the cavitation disappears, a local reaction field estimated to have a pressure of 87,000 atm and a temperature of 160,000° C. is instantaneously formed to precipitate a gas again which has been dissolved in the surrounding liquid. This phenomenon is likely to occur if the output of ultrasonic waves is excessively increased and if the degassing process speed per unit area is low.
Now, the problem 3) will be described. In the conventional ultrasonic degassing apparatus, as already described, when the cavitation disappears, a shock wave is generated which instantaneously forms a local reaction field estimated to have a pressure of 87,000 atm and a temperature of 160,000° C. Thus, the coating liquid may be damaged or piping through which the coating liquid is transferred may be corroded.
Now, the problem 4) will be described. In the conventional ultrasonic degassing apparatus, bubbles are likely to precipitate in the case of insufficient degassing and an excessive increase in the output of ultrasonic waves. To deal with this, for example, Japanese Patent Application Publication No. 1999-197406 and No. 1999-290611 and the like propose the following configurations: the one that externally transmits one or more frequency variation cycles of ultrasonic vibration to the coating liquid in a reservoir or transfer pipe; in a frequency variation cycle, vibration varies from a low frequency to a high frequency, and the one that continuously modulates the oscillation frequency of ultrasonic waves applied, over a predetermined frequency bandwidth around a frequency used as a reference to periodically continuously vary the distribution of sound pressure of ultrasonic waves applied to the coating liquid. An ultrasonic degassing system with this frequency modulating system has significantly reduced the insufficiency of degassing. However, the degassing may still be insufficient depending on the physical properties of the liquid and the degassing process speed per unit time. Thus, disadvantageously, the output of ultrasonic waves for each physical property of the liquid must be individually set in view of the degassing process speed per unit time.
Now, the problem 5) will be described. Japanese Patent Application Publication No. 1999-130547, No. 1996-318102 and No. 1999-262601 and the like propose configurations having a plurality of degassing steps obtained by combining the ultrasonic degassing method with other degassing methods. For example, according to Japanese Patent Application Publication No. 1994-130547, a coating liquid is guided to a predetermined degassing container, and the pressure of the coating liquid is reduced to float and remove air bubbles. Then, the interior of the same degassing container is pressurized, which the coating liquid is irradiated with ultrasonic waves. Consequently, air bubbles are dissolved in the coating liquid. This method has a certain degassing capability. However, since the same container must undergo a plurality of steps, the degassing process requires a long time. Further, disadvantageously, the size of the apparatus must be increased.